The main objective of our prospective double blind, randomized, controlled trial is to compare the rate of alveolar fluid clearance, a marker of alveolar edema fluid resorption, in 2 groups of patients with ARDS and suffering from hypovolemia: those treated with albumin and those treated with crystalloid (fluid challenge of 7 mL/kg over 15 minutes, that can be repeated 3 times if hypovolemia persists).